258 DEVELOPMENT OF RICCIOCARPUS 



plants (Fig. ijg, A, B). The appearance of many of these hepa- 

 tics is suggestive of the algae. Especially is this true of the aquatic 

 Ricciocarpus and Riccia. 



(a) Structure of Ricciocarpus. — An examination of the struc- 

 ture of one of these will show, however, that extensive changes 

 have been induced in even the simplest forms. The new stimuli 

 to which the terrestrial conditions expose them cause a remarkable 

 series of transformations in the cells that are cut off from the apical 



Fig. T79. Forms of semiaquatic Marchantiales: .4, Riccia, showing the 

 dichotomous branching of the thallus. B, Ricciocarpus. The sexual or- 

 gans are concealed in furrows that appear as radiating lines in the center 

 of the branches. C, diagram of a cross-section of a branch, showing the 

 male gametangia in the bottom of one of the furrows. 



cell. The upper cells of the thallus, as soon as they are formed at 

 the growing point, are exposed directly to the air and light, and 

 they develop chlorophyll and grow up into vertical rows or plates 

 just as you have already noticed in the palisade chlorenchyma of 

 the leaf (Fig. 1 80, yl ) . At an early period these rows of green cells 

 become separated so that air spaces arise between them, and thus 

 the chlorophyll-bearing cells are brought into direct contact with 

 the atmosphere and enabled to carry on photosynthesis to the best 

 advantage. The terminal cells of these rows enlarge considerably 

 and form a rudimentary epidermis. It is also interesting to note 

 that the direct contact of the cells with the atmosphere results for 

 the first time in the formation of a cuticle. In most species the 



